Photoacoustics of single aerosol droplets immobilised by counter-propagating optical tweezers

Abstract

In this contribution we discuss the influence of relative humidity on photoacoustic measurements from both an experimental and theoretical perspective. We present a refined model of the photoacoustic (PA) signal that accounts for elevated particle temperatures and different levels of relative humidity. We use this new model together with the photoacoustic data collected with our photothermal single-particle spectrometer (PSPS) to retrieve the mass accommodation coefficients of water on organic aerosol particles. The single-particle nature of our experiments is achieved by employing counter-propagating tweezers. Furthermore, we investigate the influence of relative humidity on the eigenfrequency of the PA cell.

Shining New Light on the Kinetics of Water Uptake by Organic Aerosol Particles

“…The two trapping arms (each ∼250 mW) are directed onto two facing aspherical lenses (79 mm, ASL10142 Thorlabs), which focus the beams at the center of the photoacoustic cell, creating a potential well to trap a particle. To enhance the stability of the optical trap, a feedback loop is introduced, which adjusts the respective powers in the two trapping arms. ,, This is achieved by monitoring the particle’s position using a quadrant photodiode (Hamamatsu, S5980) and changing the polarization of the trapping laser prior to the polarizing beamsplitter cube using an electro-optic modulator (EOM, 350-50-01, ConOptics), driven by a proportional–integral–derivative (PID) controller.…”

“…To enhance the stability of the optical trap, a feedback loop is introduced, which adjusts the respective powers in the two trapping arms. 77,202,204 This is achieved by monitoring the particle's position using a quadrant photodiode (Hamamatsu, S5980) and changing the polarization of the trapping laser prior to the polarizing beamsplitter cube using an electro-optic modulator (EOM, 350-50-01, ConOptics), driven by a proportional−integral−derivative (PID) controller.…”

“…In our most recent work we have presented an extended model which accounts for elevated average particle temperatures. 204 Briefly, the photoacoustic signal (S) originates from the heat (Q) and mass (I) flux from the particle to the gas phase and can be described as i k j j j j j j y…”

“…Figure 5 shows the mass accommodation coefficient of water on tetraethylene glycol retrieved from the PAA, PAP, and MMS using the data from refs 77, 202, and 203 and our extended model. 204 Overall, nearly 400 single-particle measurements were taken to verify reproducibility and ensure statistically significant data collection. The mass accommodation data is presented for different particle sizes (lower abscissa), which correspond to different particle temperatures (upper abscissa).…”

“…As a result, PA cells have to be properly characterized before performing RH-dependent measurements. 204 Furthermore, accurate size determination (to a few nanometers) of the trapped aerosol particles is essential in order to obtain good fits to the modulated Mie scattering spectra. These issues are in principle solvable but require a high degree of control over the experimental setup.…”